The present invention relates to phantoms for use with radiation therapy equipment and in particular to a phantom providing improved quality assurance measurement of radiation therapy machines operating in a rotational mode.
External beam radiation therapy treats cancer in a patient by means of variable energy radiation beams directed through a patient's skin to the site of the tumor. Normally, the tumor is treated with number of beams, each at different angles, the beams converging on the tumor site to increased effect while reducing exposure at the skin and other normal tissue in the path of the beams.
Intensity modulated radiation therapy (IMRT) machines may employ an increased number of beams to provide a uniform dose the tumor. To increase the uniformity and the conformity of the dose to the target, arc beams are used to deliver radiation from a throughout a continuous range of angles, as in the case of conventional radiation treatment machines, or in a helical fashion as used by the Hi-Art tomotherapy machine. The intensity of multiple rays within the beam is varied over the angular range to produce a radiation dose distribution that conforms closely to tumors especially for targets of complex shapes.
It is important that all radiation therapy machines be checked regularly to ensure that the correct dose is provided for treating the tumor. Daily quality assurance measurements may be performed by placing a radiation detector such as an ionization chamber on the patient table of the radiation therapy machine and directing the radiation beam downward toward the ionization chamber. The beam is directed through blocks of attenuating material approximating the characteristics of tissue, and measured with two thicknesses of attenuating material to deduce radiation energy. The fluence and energy are compared to target values for the machine.
The angle of the radiation beam can affect the beam characteristics as an indirect result of variations in gravitational load on the machine components including the linear accelerator. The increased dose accuracy and large number of angled beams used in IMRT makes measurement of the beams at other than vertical angles and while the beam is rotating important. Normally, the characteristics of the treatment beam are measured with the beam in a vertical position.